The present invention relates to reciprocating compressors, in particular to an actuator for controlling the delivery rate of reciprocating compressors and, more specifically, an electromechanical actuator for the continuous return flow control of the suction valve(s) of reciprocating compressors, i.e. an electromechanical actuator capable of controlling the suction valve on each compression cycle.
Return flow control is one of the known systems of flow control in reciprocating compressors and is implemented by delaying the closure of the suction valve(s) with respect to the point of closure in the case of maximum flow. The gas entering the cylinder of the compressor flows back in the suction line by an amount proportional to the length of the compression stroke during which the suction valve(s) are kept open.
It is wished to point out that in the following description, reference will be made to “dynamic” seals, washers or sealing elements and to “static” seals, washers or sealing elements. The term “dynamic” sealing elements means sealing elements that are subject to wear because they are positioned between two parts of which at least one is in movement, and are therefore elements subjected to friction and dynamic wear, while “static” sealing elements means sealing elements located between two fixed parts, and therefore not subject to wear resulting from movement.
In quite recent times, several different return flow control devices constituted by electromagnetic actuators and intended for reciprocating compressors have been implemented in the state of the art, these including the following patents: WO2008000698, in the name of the applicant and relating to an apparatus for the continuous regulation of reciprocating compressors; U.S. Pat. No. 7,651,069, relating to a valve comprising an electromagnetic actuator; US 2012/0260796, relating to a reciprocating compressor with delivery rate control; and WO2011009879, relating to a method for controlling the delivery rate of a reciprocating compressor and a reciprocating compressor with delivery rate control.
Normally, regulating devices of an electromagnetic type are provided with electromagnets that set in motion a moving element cooperating with a rod of the actuator, in turn cooperating with the suction valve(s) of the compressor, and must often operate at many different pressures and with many different types of gas, many of which are highly flammable, such as refinery gases.
One of the major issues to solve in these devices is to prevent compressed gas from some way coming into contact with the windings of the electromagnets or with the associated electronic devices, such as the position sensor of the moving element for example; in fact, a mixture composed, for example, of oxygen present in the air and an flammable gas, can explode if exposed to an ignition source of electrical origin.
In the current state of the art, these electromechanical actuators and, more generally, all actuators used for reciprocating compressors, whether pneumatic or hydraulic, are provided with dynamic seals positioned on the rod of the actuator; the reciprocating compressor described in document US20100086415 may be mentioned by way of example.
Unlike normal actuators used for return flow control with stepwise flow control, actuators used in return flow control capable of providing continuous flow control are subjected to extremely high activation frequencies; by way of example, it is sufficient to consider that a compressor rotating at 600 rpm, if continuously return flow controlled, would require roughly 315 million activation cycles of the actuators in a year, i.e. 315 million downward displacements of the actuator's armature and associated rod and the same number of upward displacements. Since the actuator's rod slides inside an associated seat where the annular sealing elements are located, one of the biggest problems with this type of actuator is the wear on these sealing elements, and hence the risk that compressed gas enters the chambers housing the armature and so makes contact with the windings of the actuator's electromagnets.
To reduce the risk of compressed gas returning to these chambers, precisely in the event of wear on the rod's seals, it is known to use systems of recovering gas through the rod and to use nitrogen barriers or barriers of equivalent inert gases that reduce the likelihood, but which are not found to be entirely efficient.
These sealing elements along which the actuator's rod slides can also be damaged by possible impurities or metallic particles that might be present in the compressed gas.
Furthermore, unlike pneumatic or hydraulic actuators, a gas leak in electromechanical actuators from the rod of the actuator, following possible damage to the dynamic sealing elements of the rod, implies high safety risks for the entire compressor, as, in cases of easily flammable gases such as, for example, hydrogen, ethylene and, more generally, the various hydrocarbons, the gas would enter the chambers housing the electromagnets.
As is known, to prevent possible infiltrations of compressed gas affecting the windings of the electromagnets, resin finishing processes are applied to them, but this resin finish is not sufficient to ensure there are no infiltrations of the compressed gas. It should also be considered that the possible formation of air pockets inside or beneath the resin finish and gas infiltrations jeopardise not only the actuator, but the compressor itself. In fact, it should be remembered that reciprocating compressors can operate at many different pressures, with suction pressures that can even exceed 100 bar.